1. Field of the Invention
This invention relates to a branching filter (distributor) for a transmitter-receiver, which has a transmitter port for receiving an input transmit signal, a receiver port, and an antenna port for receiving an input receive signal and is for distributing the input transmit signal to the antenna port and the input receive signal to the receiver port.
2. Description of Related Art
A branching filter for a transmitter-receiver is known as being used to commonly use an antenna for transmission and reception for using a microwave band. A conventional branching filter for a transmitter-receiver comprises a transmission filter, a waveguide branching filter coupled to the transmission filter, a bend waveguide coupled to the waveguide branching filter, and a reception filter coupled to the bend waveguide. In the conventional branching filter for the transmitter-receiver, it is impossible to easily and cheaply produce the branching filter, and it must be designed in larger size because the transmission filter, the reception filter, the waveguide branching filter and the bend waveguide filter are separately fabricated.
On the other hand, a transmitter-receiver comprises a transmitter module, a branching filter module coupled to the transmitter module, and a receiver module coupled to the branching filter module.
In a conventional transmitter-receiver, a transmitter module comprises a transmitter connector, and a receiver module comprises a receiver connector. On putting the transmitter-receiver in operation, a cable is connected to the transmitter connector and the receiver connector. Points of connection of the cable to the transmitter connector and the receiver connector are covered for hermetic seal and for insuring waterproofness by a first and a second connector cover. It is hardly possible in the conventional transmitter-receiver to exchange the first and the second connector covers to new covers.
FIG. 4 is an exploded perspective view showing a conventional separate type branching filter for a transmitter-receiver as disclosed in U.S. Pat. No. 5,243,306. This branching filter for the transmitter-receiver was proposed to overcome the "large-size structure problem" of the branching filter for the transmitter-receiver as described above.
As shown in FIG. 4, the conventional branching filter for transmitter-receiver basically comprises three parts, that is, a first part 101, a second part 102 which is detachably coupled to the first part 101 and a third part 103 which is detachably coupled to the second part 102. The first part 101 includes an antenna port, a part of a waveguide branching filter (orthogonal transducer) and a part of a reception filter. The second part 102 includes the residual (other) part of the waveguide branching filter, the residual (other) part of the reception filter and a part of a transmission filter. The third part 103 includes the residual (other) part of the transmission filter. The reception filter which is assembled by the first and second parts 101 and 102 is provided with a bend waveguide 104 to facilitate a connection work between the branching filter and a transmitter-receiver.
FIG. 5 is a cross-sectional view showing an assembly of the first, second and third parts 101, 102 and 103. The assembly is designed in a dimension of about 160 mm (height).times.80 mm (width), for example. FIG. 6 is a cross-sectional view showing partially the third part 103 shown in FIG. 4. The third part 103 is designed in a dimension of about 70 mm (width).times.65 mm (height), for example.
In the conventional branching filter for transmitter-receiver as described above, each of the waveguide branching filter, the transmission filter and the reception filter is divided into plural parts, and these plural parts are fabricated into a waveguide. Therefore, if each part is manufactured with a dimensional error or fabricated with a positional error, a step portion would occur at a divisional (partitional) face portion of the finally-fabricated waveguide (i.e., at the boundary between the parts constituting the waveguide) because of the dimensional or positional error).
If a step portion occurs at the divisional face portion of the waveguide of the waveguide branching filter, cross-polarized discriminating power (XPD) is deteriorated. Further, if a step portion occurs at the divisional face portion of the waveguide of each of the transmission filter and the reception filter, intrinsic impedance of each filter is deteriorated. In this case, the impedance of each filter is deviated from a predetermined impedance to induce reflected wave or standing wave, resulting in occurrence of transmission loss.
Further, since the branching filter for transmitter-receiver is constructed by assembling plural parts, fixing members for fixing the respective parts are required, and thus the branching filter for a transmitter-receiver must be designed in large size.
Still further, as described above, the bend waveguide is provided to the reception filter side. Therefore, in a case where the bend waveguide is disposed between the reception filter and the reception connection port, the branching filter for the transmitter-receiver must be designed in large size. On the other hand, in a case where the bend waveguide is disposed between the reception filter and the waveguide branching filter, the total length of the branching filter and a transmitter-receiver when the transmitter-receiver is connected to the branching filter is excessively large.